Current gas turbine engines continue to improve emissions and engine efficiencies. Notwithstanding these improvements, further increases in engine efficiencies will require finer balancing of NOx and carbon monoxide (CO) emissions to meet increasing regulations. Some regulations include limits of 5 ppmv NOx and 10 ppmv CO.
Reducing production of NOx and CO many times require conflicting operating conditions. NOx is an uncertain mixture of oxides of nitrogen generally produced when an excess of atmospheric oxygen oxidizes nitrogen. NOx production typically increases as a flame temperature in a combustor increases. In contrast, CO production increases as the temperature in the combustor decreases. At temperatures above 1800 F. (982 C), CO reacts with excess oxygen to form carbon dioxide (CO2). CO2 is generally considered an unobjectionable emission. Like CO emissions, gas turbine efficiencies generally improve with increasing flame temperatures. However, most materials currently used in gas turbine engines exhibit reduced durability above an upper temperature limit.
Decreasing NOx production in gas turbine engines typically involves reducing the flame temperature. One such example involves injecting water or steam into the combustor. Water injection reduces flame temperatures but may increase wear and corrosion in the turbine. Also, water injection requires additional hardware including water storage tanks, water pumps, and water injectors. Lean premixed combustion attempts to decrease NOx production while maintaining engine efficiencies. A lean premixed combustor premixes a quantity of air and a quantity of fuel upstream of a primary combustion zone. Increasing the quantity of air introduced upstream of the primary combustion zone reduces the flame temperature similar to the introduction of water. By reducing the flame temperature, NOx production also decreases.
Even with the reduced flame temperature, a combustor liner wall near the primary combustion zone requires cooling to increase its durability. A film of cooling air typically flows generally parallel to a hot side of the combustor liner wall in the primary combustion zone. This film protects the combustor liner wall by forming an insulating layer of cool air along the combustor liner wall. However, this film tends to quench the flame along the combustor liner wall. As the flame quenches at the combustor liner wall, CO reactions with excess oxygen to form CO2 retard. Unreacted CO enters an exhaust stream and contributes to the overall emissions from the engine.
U.S. Pat. No. 5,636,508, issued to Shaffer et al. on Jun. 10, 1997 describes a ceramic combustor liner. Ceramic materials generally tolerate higher temperatures than a metal combustor liner. A typical ceramic liner may reach temperatures near 2000 F. (1093 C). In comparison, metal combustor liners typically operate at temperatures up to 1550 F. (843 C). However, many ceramic and metallic combustor liners require cooling to improve their operational life. Metallic liners often cool a cold side (backside) of the combustor liner. Typical methods usually incorporate impingement cooling or protrusions into cooling channel. Both of these methods result in pressure reduction of the air in the cooling channel. With this reduction in pressure, the air from the cooling channel may not be used as combustion air (primary air). Instead, the air from the cooling channel is used as dilution (secondary) air to assist in regulating a gas temperature profile at the combustor outlet.
U.S. Pat. No. 5,575,154, issued to Loprinzo on Nov. 19, 1996, describes a dilution flow sleeve to reduce CO emissions. The dilution flow sleeve improves emissions by increasing the mixing of the film cooling flow along a hot side of the combustor liner wall with a core combustion region. The increased mixing of flow downstream of the primary combustion zone improves the reaction of CO with excess oxygen to form CO2. Air introduced into the dilution flow sleeve enters the combustor downstream of the primary combustion zone. To adequately reduce NOx, cooling air generally must be introduced into the primary combustion zone to reduce flame temperature.
The present invention is directed at overcoming one or more of the problems set forth above.